Conventional electronic equipment has a number of keys for performing various setup and switching of operation with respect to the equipment at a front or a side thereof. In such electronic equipment, when any key is operated, a signal (voltage signal) corresponding to the key is supplied to, for example, a microcomputer provided in the electronic equipment. The microcomputer identifies the operated key based on the supplied signal (voltage signal) and controls the operation of the electronic equipment based on a result of identification.
Patent literatures No. 1 to No. 3 mentioned below describe common means for identifying the operated key based on the signal (voltage signal) supplied to the microcomputer.
Patent literature No. 1 describes a key matrix circuit. In this circuit, when any selection key is pressed in a keyboard with a plurality of selection keys arranged in a matrix, two pairs of voltage levels corresponding to the pressed selection key are detected, and the pressed selection key is identified based on the detection result.
Patent literature No. 2 describes a key switch circuit comprising a plurality of switch groups and a hold switch for holding an operation command by a key switch. The key switch circuit is provided with a hold circuit. When no key switch in the key switch groups is operated, the hold circuit holds a voltage at a particular value different from a value of an output voltage generated in the key switch circuit, whereby it is capable of determining whether the key switch circuit is connected or not, without increasing the number of input ports for such as CPU (Central Processing Unit).
Patent literature No. 3 describes a key input device. In this key input device, each one end of a plurality of resistor string parts to produce the resistance value corresponding to the input key is connected electrically to a power supply, and a selection part electrically connects one of the plurality of resistor string parts to ground in response to a selection command from a control unit. Further, one of the resistor string parts corresponding to the selection command from the control unit is electrically connected to the power supply and, when a value of the voltage corresponding to the input key at the resistor string part is generated at a connecting node, the control unit identifies the input key based on the value of the voltage generated at the connecting node and the selection command given to the selection part.